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Thread: Tethers commonly break 1 to 2 years after VBT procedures

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    Tethers commonly break 1 to 2 years after VBT procedures

    Anterior Spinal Growth Tethering for Skeletally Immature Patients with Scoliosis
    RESULTS: Eight (47%) of the patients had a suspected broken tether.
    From the study of 17 patients:
    Broken Tether Suspected: 8 patients
    1 broken level: 7 patients
    3 broken levels: 1 patient

    Location of broken levels:
    T8-9: 4
    T9-10: 1
    T10-11: 3
    T11-12: 1
    T12-L1: 1

    Time period of tether breakage:
    1st erect to 6 months postop: 0
    6 months to 1 year postop: 0
    1 year to 18 months postop: 1
    18 months to 2 years postop: 3
    2 years to 2.5 years postop: 2
    2.5 years to 3 years postop: 1
    3 years postop: 1

    Commentary:
    Previous studies have shown that the PET cord has a tensile strength of 3,000 N, yet in the current study, the tether failed in nearly half of the cases. ASGT is proposed to work via initial tensioning on the convex side, with subsequent spinal growth creating further tension until convex vertebral end-plate growth is inhibited via the Hueter-Volkmann law. Tether breakage typically occurred at the levels below the apex, with the most common broken levels being T8-9 and T10-11 (range, T8-9 to T12-L1). Interestingly, none of the cases had 3 adjacent screws simultaneously splayed due to failure. This would likely be the case if the tether failed beneath the locking set screw, an obvious stress riser. In the 2 cases of tether failure confirmed intraoperatively, the breakage occurred between the screws, not under the set screws. Until the screw angulation data were analyzed, the increased divergence of the screws and implant failures were not recognized except in the most severe example. Considering this, it is important to measure the segmental angle between each pair of screws, as widening of this angle appears to be an early indication of implant failure. In some cases, recognition of tether breakage may actually prevent further intervention. In at least 1 case in the current study, there was initially some concern about overcorrection prior to the observed widening between the screws at 1 level. After tether breakage, the progression of the correction slowed to the point that implant removal for overcorrection was unnecessary. The exact cause of the failure remains unclear; however, because of the greater motion in the lower thoracic region, this area seems to be at higher risk. A more robust implant would be ideal, although an implant with a fatigue life of decades rather than years seems unlikely. Further development is necessary.
    Last edited by Dingo; 10-04-2018 at 12:56 PM.

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